Most systems used for imaging light or other electromagnetic radiation, including the human eye, have little or no sensitivity to the polarization of the light or other electromagnetic radiation. In some cases, such as the case of polarizing sunglasses, an external filter with specific polarization properties is used in order to provide benefits, such as suppression of the glare from road surfaces, especially wet road surfaces, viewed from a motor vehicle.
With the exception of a light reflected from a few rare species of beetles, there are no significant natural sources of light or other electromagnetic radiation that is circularly polarized. Therefore, manmade objects from which circularly-polarized light emanates can be detected and recognized based on this polarization characteristic, without the need for evaluating any other characteristic.
With available technology, it has become possible to produce imager systems that allow the visualization of differential circular polarization in imaged light or other electromagnetic radiation. It has also become possible to produce paints, pigments, or other treatments of objects such that when such objects are illuminated with unpolarized light or other unpolarized electromagnetic radiation, the electromagnetic radiation emanating from these objects is fully or partially circularly polarized.
There are many potential applications of objects that have differential circular polarization reflectance, together with imagers that can be used to allow the detection of such differential circular polarization reflectance. One such potential application is the detection of marked objects in a cluttered scene. Harrowing descriptions of the difficulty of searching for survivors of accidents at sea make it obvious that improvements in the ability to detect objects such as life jackets on the ocean surface could mean the difference between life and death for such survivors. Life jackets with differential circular polarization reflectance, when viewed with appropriate equipment, would be much easier to detect than conventional life jackets, even conventional life jackets with distinctive coloring. In entertainment applications, it could be useful to be able to make selected objects appear to pulse or blink without any such pulsing or blinking of the scene within which the selected objects are embedded. In tagging or marking of products or currency, it would be beneficial if tags or marks could be made visible with special equipment, even though the tags or marks are not visible without such special equipment.
At the present time, there is no system or method available for purchase that provides the combination of such objects with differential circular polarization reflectance along with the associated viewing or imaging equipment.
FIG. 1 illustrates a principle known from prior art that allows the creation of paints, pigments, or treatments that result in the light emanating from a treated object to be circularly polarized over a broad spectral bandwidth when the object is illuminated with unpolarized light. To produce this desired broad spectral bandwidth, a chiral material such as a cholesteric liquid crystal polymer (CLCP) film is fabricated in such a way that the pitch of the chiral structure comprising the film has a gradient in the direction perpendicular to the surface of the film.
In the example shown in FIG. 1, a chiral structure of a CLCP or other transparent anisotropic material results in reflection of one circular polarization and transmission of the other circular polarization. The pitch of the chiral structure is small near the surface of the film, resulting in the reflection of radiation of one circular polarization in the blue region of the spectrum. The pitch of the chiral structure is moderate in the middle of the film, resulting in the reflection of radiation of one circular polarization in the green region of the spectrum. The pitch of the chiral structure is large at the back of the film, resulting in the reflection of radiation of one circular polarization in the red region of the spectrum. An absorber behind the film absorbs light with the circular polarization that is not reflected.
The result of illuminating a film such as the one illustrated in FIG. 1 with unpolarized light is that the light reflected from the surface is circularly polarized over a broad spectral band. Thus, with materials of this type, paints, pigments, and other treatments can be fabricated such that treated objects emanate circularly-polarized light over a broad spectral band.
Thus, there is a need for systems and methods for producing objects with differential circular polarization reflectance, together with equipment that allows the detection and visualization of such objects.